1. Field of the Invention
The present invention relates to a die plate for resin granulation, and more particularly, to a novel improvement for ensuring quality of pellets by eliminating temperature differences among extrusion nozzle holes with use of a heating medium flowing through a heating channel only once and flowing out through a heating medium outlet.
2. Description of the Related Art
In general, a granulation apparatus for obtaining plastic pellets is disposed downstream of an extrusion machine, and includes a die plate having nozzles through which molten resin is extruded, and a cutter device having a cutter blade for cutting the resin extruded from the die plate nozzles into pellets. A surface of the die plate on a side on which the resin is extruded is exposed to circulating water for delivering the cut pellets. Accordingly, in order to maintain the temperature of the resin extrusion nozzles of the die plate, jackets are built therein, through which a heating medium, such as steam or heat transfer oil, flows. In order to perform granulation for obtaining pellets having uniform shapes, it is desired that all of the molten resin extrusion nozzles of the die plate be uniformly heated. Therefore, the heating jackets and the extrusion nozzles are disposed with a given regularity. The heating jacket includes an outer circumferential annular heating passage, an inner circumferential annular heating passage, and heating channels for connecting the outer circumferential annular heating passage and the inner circumferential annular heating passage to each other. The heating channels may be disposed parallel to one another, for example, in a range of ¼ of the circumference (i.e., 90 degrees), ⅙ thereof (60 degrees), or ⅛ thereof (45 degrees).
As an example of a conventionally used die plate for resin granulation of this kind, there is a structure disclosed in Japanese Patent Application Laid-open No. Hei 11-277528, which is illustrated in FIG. 6.
In FIG. 6, a plate main body 1 of a die plate 1A includes an otter annular heating passage 5 formed on an outer circumferential edge of this plate main body 1, and an inner annular heating passage 4 formed on an inner side of this outer annular heating passage 5 so as to have a smaller diameter than that of the outer annular hefting passage 5. The inner and outer annular heating passages 4 and 5 are coaxially disposed.
The outer annular heating passage 5 is partitioned by a plurality of outer walls 5A into first to fourth regions A, B, C, and D with an angular range of 90 degrees. The inner annular heating passage 4 is partitioned into two parts by a pair of inner walls 4A with an angular range of 180 degrees.
In the first and fourth regions A and D of the outer annular heating passage 5, a pair of heating medium inlets 2 into which a heating medium F flows are formed, and, between the second and third regions B and C of the outer annular heating passage 5, one heating medium outlet 3 is formed.
In the respective regions A to D between the outer annular heating passage 5 and the inner annular heating passage 4, nozzle hole groups 7 including a large number of extrusion nozzle holes 6 are disposed. Each of the nozzle hole groups 7 is surrounded by the heating channels 8 and the annular heating passages 4 and 5.
The respective extrusion nozzle holes 6 of each of the nozzle hole groups 7 are disposed in two rows, including a first row 6A and a second row 6B, along a crossing direction E which crosses the circumferential direction of the plate main body 1.
Other than the above-mentioned conventional configuration, there is another conventional configuration as illustrated in FIG. 7.
Specifically, in FIG. 7, portions identical or equivalent to those of FIG. 6 are represented by the same reference symbols, and description thereof is omitted. The regions A to D are respectively formed as closed circuits. The heating medium inlet 2 and the heating medium outlet 3 are formed in each of the outer annular heating passages 5. The nozzle hole groups 7 of each of the regions A to D are partitioned into two parts by the outer wall 5A.
In the above-mentioned conventional configurations, first, in the conventional configuration of FIG. 6, the heating medium F, which has been preheated to a given temperature and supplied through the heating medium inlets 2 in the regions A and D, passes through the heating channels 8 in the regions A and D, and heats the extrusion nozzle holes 6. Moreover, the heating medium F is delivered to a heating device (not shown) through the single heating medium outlet 3 on the lower side after passing through the heating channels 8 in the regions B and C. Then, the heating medium F is heated again to the given temperature, and after that, circulated again into the plate main body 1 through the heating medium inlets 2.
Next, in the another conventional configuration illustrated in FIG. 7, the nozzle hole groups 7 are partitioned into two sections by the outer wall 5A for each of the regions A to D. Accordingly, as indicated by the arrows in each of the regions A to D, the heating medium F, which is supplied through the heating medium inlet 2, heats the plurality of nozzle hole groups 7 in the vicinity of the heating medium inlet 2. After that, the heating medium F heats the plurality of nozzle hole groups 7 in the vicinity of the heating medium outlet 3, and then the heating medium F is returned to the heating device (not shown) through the heating medium outlet 3.
In the conventional die plate for resin granulation configured as described above, the following problems exist.
That is, in the case of the above-mentioned conventional configuration of FIG. 6, the heating medium flows into the outer annular heating passage through the inlets disposed at two upper positions. The heating medium, which has flowed from the upper-right inlet, flows into the heating channel which communicates to the outer annular heating passage and is disposed in the upper-right range of 90 degrees. The heating medium is used for heating the nozzles there, and then flows into the inner annular heating passage. The same heating medium flows into the inner annular heating passage which is disposed in the lower-right range of 90 degrees, and then flows into the heating channel which is disposed in the lower-right range of 90 degrees. The heating medium is used for heating the nozzles, then passes through the outer annular heating passage and flows out through the lower heating medium outlet. In other words, the same heating medium passes through the heating channels twice (to be used for heating the nozzles twice) in this configuration. The heating medium performs heat transfer for heating the nozzles when passing through the heating channels. Therefore, the temperature of the heating medium itself is lowered (the heat energy stored therein is decreased). When the heating medium having the temperature thus lowered is caused to pass through the heating channels again for heating the nozzles, a temperature difference is generated between the vicinity of the heating channels through which the heating medium has passed firstly, and the vicinity of the heating channels through which the heating medium passes secondly. In this example, the temperature of the nozzles in the lower-right range of 90 degrees is lower than the temperature of the nozzles in the upper-right range of 90 degrees.
Reduction of the variations in size and shape of the pellets is an important factor for stabilizing the formability in a succeeding process. In order to obtain uniform pellet shapes having small variations, it is important to minimize the temperature differences among all the nozzle holes in the die plate. When there are variations in temperature of the nozzle holes, the flow rate of a resin passing through the hole having a lower temperature becomes lower so that the size of pellet becomes smaller. On the other hand, the flow rate of a resin passing through the nozzle hole having a higher temperature becomes higher so that the size of pellet becomes larger. In particular, when the size and capacity of the granulation machine become larger, the number of nozzle holes becomes larger so that uniform heating of the nozzle holes becomes an important subject.
In the case of the conventional configuration of FIG. 7, the inlet and outlet for the heating medium are formed in each of the regions, and the heating medium, which has undergone heat exchange with the nozzle hole groups on the inlet side, is recirculated to the nozzle hole groups on the outlet side. Accordingly, there are generated temperature differences between the nozzle hole groups on the inlet and outlet sides so that there are generated variations in shape of molded pellets similarly as described above.